For example, at regular inspection of an atomic power plant, an automatic instrument is placed in a reactor to get access to devices in the reactor and various maintenance methods are executed. The methods include laser peening as a technique capable of effectively preventing the occurrence of SCC (Stress Corrosion Cracking) resulting from a tensile stress remaining at a welded portion.
A principle of the laser peening will be described. Laser light having a pulse width of about several nanoseconds (ns) is condensed to a spot of a diameter of an about 1 mm by a condensing lens to be irradiated to a member to be treated (target member). As a result, a surface of the member to be treated absorbs energy to form into plasma. In the case where the periphery of the plasma is covered with liquid or paint having a transparency with respect to a wavelength of the laser light, expansion of the plasma is hindered. Consequently, internal pressure of the plasma reaches about several gigapascals (GPa) to make an impact on the member to be treated. In this event, a strong shock wave occurs and propagates in the material to cause plastic deformation and change the residual stress into a compressed state.
The laser peening less depends on a material strength and is capable of processing the member to be treated up to its inner part with a plate thickness of about 1 mm from a surface of the member to be treated, as compared with another peening technique such as a shot peening, a water-jet peening or the like. Further, the laser peening provides excellent processability at a confined portion because there is no reaction force during the processing and its processing apparatus can be easily downsized.
In the conventional laser processing apparatus used in a pipe, a processing target portion is desirably in an underwater environment, but the processing is sometimes forced to be executed in gas. For example, when a processing target is not sunk very deep in a water tank, the processing target in the gas is irradiated with laser light while being supplied with liquid such as water. However, when the supply of the liquid is stopped or its supply amount reduces due to some trouble, there is a possibility that the laser light that has propagated in the gas is irradiated to the processing target. When the laser light propagates in the gas, a focal length changes as compared with the case where it propagates in water. In this case, there is a possibility that the laser light is converged to be irradiated to the processing target. Since the focused laser light has a high energy density, there is a possibility that the laser light damages the processing target portion in the worst case.